Purpose: To determine the safety and efficacy of a single in vivo gene transfer of the Herpes simplex thymidine kinase gene by infiltrating the surgical resection margin of the tumor with escalating doses of PA317/G1Tk1SvNa.7 vector producer cells followed 14 days later by a two week course of ganciclovir in pediatric patients with progressive or recurrent malignant supratentorial brain tumors.